1. Technical Field
The present invention relates to data recovery devices, and more particularly, to a data recovery device for recovering transmitted data by tracing timings of a transmission signal and detecting symbols on data streams of the signal in a forward control channel of a mobile telephone system.
2. Description of Related Art
In an advanced mobile telephone system (AMPS), which is an analogue cellular mobile telephone system, the processes of set-up and hand-over (or hand-off) speech paths (or traffic channels) are performed through data transmission/reception between a base station and a mobile station (or a terminal). For example, an analogue cellular telephone system sets up a speech path by commands from a base station and responses from a mobile station.
The AMPS protocol comprises transmitting control data from a base station to a mobile station (or terminal) through a forward control channel (FOCC or paging channel). The control data is converted into binary digital data using an NRZ (Non-Return-to-Zero) pattern. The binary digital data is converted into digital data streams encoded in a Manchester format, thereby being synchronized with timing clocks (or bit clocks), before being RF—transmitted through an air space (or a free space).
FIG. 1 shows a structure of an NRZ (Non-Return-to-Zero) data stream on a FOCC. An NRZ data stream comprises a 10-bit dotting portion, a 11-bit synchronizing portion, and a plurality of blocks of 44-bit data in a predetermined series. The series of 44-bit data blocks has two data types for service systems A and B, and the 44-bit data blocks for A and B service systems are alternatively provided to the data stream five times. The data stream further comprises busy/idle bits supplied thereto by ten bits.
FIG. 2 is a timing diagram that illustrates signal waveforms for the transmission/recovery of a signal on a FOCC or paging channel. Referring to FIG. 2, a base station encodes an original signal having the NRZ data stream into a Manchester format, synchronously in response to a bit clock of 10 KHz. The Manchester-encoded signal is modulated to form a carrier wave signal, and the modulated signal is transmitted to a mobile station through air. The mobile station demodulates the carrier wave signal, and converts the demodulated signal into the original signal having the NRZ data steam by timing and tracing the demodulated signal based on a sampling period, for example, 40 KHz. The conversion from the demodulated signal to the NRZ data stream typically utilizes sampling operations with four level points a, b, c, and d, all of which are the same absolute value within one cycle period of the synchronous signal (a=b=c=d).
FIG. 3A shows waveforms of a synchronous signal (Sync) of a demodulated signal to be recovered according to a conventional data recovery protocol. FIGS. 3B–3E show synchronous signals of the demodulated signal illustrating a soft decision for recovering data. As shown in FIG. 3B, when the demodulated signal transitions from a positive domain to a negative domain, a bit of NRZ data corresponding to the signal is set to “0” (logically low in binary) if a summation result of the level point values is positive (>>0). As shown in FIG. 3D, when the demodulated signal transitions from the negative domain to the positive domain, a bit of the NRZ data corresponding to the signal is set to “1” (logically high in binary) if a summation result of the level point values is negative (<<0).
However, a conventional data recovery protocol may cause an error in tracing and sampling the demodulated signal using a synchronous signal, if the demodulated signal has a phase shift (e.g., to left) at a certain time, Tps, such as shown in FIG. 3A during the signal transmission through the air interface.
Assume that the demodulated signal is sampled by 40 KHz in every cycle period of the synchronous signal (Sync). If the phase shift of the demodulated signal occurs at cycle period P9 (in FIG. 3A), the synchronous signal at cycle period P9 may have abnormal level points a′, b′, c′, and d′, as shown in FIGS. 3C and 3E. In FIG. 3C, a summation value of the level points, i.e., {(−a′)}−{b′+c′+d′}, results in a negative value, and thus the bit of the NRZ data corresponding to cycle period P9 is erroneously set to “1”. In FIG. 3E, the summation value of the level points, i.e., {a′}−{(−b′)+(−c′)+(−d′)}results a positive value, and thus the bit of the NRZ data corresponding to cycle period P9 is erroneously set to “0”.
The abnormal sampling of the demodulated signal using the synchronous signal typically occurs due to fading effects that disaccord synchronization states of the demodulated signal. Thus, the data recovery process at a communication system is not only an important factor in evaluating the performance of data processing in the system, but also an important factor in affecting an error bit rate (BER) thereof. Thus, a need exists for a data recovery device that can precisely and efficiently trace synchronization timings of a demodulated signal during an idle state to improve a data processing performance in an AMPS, for example.